The present invention is concerned with MPEG decoding technology, and more specifically relates to an apparatus and method for detecting a letter box for an MPEG decoder.
FIGS. 1A through 1C illustrate a letter box processing feature whereby video signals of a 16:9 aspect ratio are converted into video signals of a 4:3 aspect ratio in an encoder, and an complementary feature where video signals of a 4:3 aspect ratio, containing a letter box, are converted into video signals of a 16:9 aspect ratio.
Most existing televisions are configured for display in the 4:3 aspect ratio. Accordingly, images for display that have the 16:9 aspect ratio as shown in FIG. 1A are first converted into images of 4:3 aspect ratio as shown in FIG. 1B, which is referred to as an MPEG encoding operation. In this case, in order to match the different aspect ratios of images to each other, a standard of MPEG-2 referred to as Pan-Scan is used or images are encoded by inserting invalid image data into upper and lower areas of images, for example into letter box regions a and a′.
However, in the case where the image shown in FIG. 1B is decoded and injected on a screen having a 16:9 or 4:3 aspect ratio, while the image matches with the aspect ratio of the 4:3 screen, an image mismatch occurs with the aspect ratio on the 16:9 screen, so that left and right regions of the screen do not correspond to pixel values of decoding image. Accordingly, a screen without left and right regions b and b′ is displayed or an image is injected thereon after interpolation, resulting in an unnatural image pattern. Also, since ineffective images appear at the upper and lower regions, a and a′, or appear at the left and right regions, b and b′, space on the screen display is wasted. In addition, for a television employing a CRT (Cathode-Ray Tube) as a monitor, a same pixel value is continuously output, which can cause screen damage.
In this case, when a decoded image is displayed on a screen, if images a and a′ inserted upon encoding are removed, and therefore an original 16:9 image is injected on the 16:9 screen, it is possible to watch an image that has not yet been encoded in its original state. However, since it is impossible for the MPEG decoder to identify the size of the image inserted upon encoding the MPEG, the resulting size of the image is selected by a user in accordance with his own reference using a predetermined size. Furthermore, as the numerical value is incorrect, this technique is ineffective in image reproduction. Therefore, in order to display a correct image in accordance with the aspect ratio of the screen, it is important to correctly detect the size of the letter box a and a′ inserted into the upper and lower regions upon encoding.